1. Field of the Invention
The present invention relates to surface structural analysis of materials and structures and structural analysis in a depth direction from surfaces. Particularly, the present invention relates to a structural analysis method using cluster ion bombardment and a measuring apparatus therefor.
2. Description of the Related Art
As a surface analysis method and apparatus, a generally used method of analyzing surface structures uses a photoelectron spectrometer, an X-ray microanalyzer, an Auger electron spectrometer, or a time-of-flight secondary ion mass spectrometer.
The time-of-flight secondary ion mass spectrometer (referred to as “TOF-SIMS” hereinafter) is an apparatus in which a sample surface is bombarded with primary ions such as Ga+, In+, or Au+ in a vacuum to ionize the constituent elements and molecules of the sample surface, and the times of flight of the emitted secondary ions are measured to obtain a mass spectrum of the constituent elements and molecules of the sample surface. Japanese Patent Laid-Open No. 2004-219261 discloses an example in which a gradient shaving surface of a thin film was analyzed by TOF-SIMS. The TOF-SIMS is advantageous in that elements and molecules of a sample surface can be detected with high sensitivity.
In order to analyze a structure in the depth direction from a surface thereof, a generally used method is to analyze the structure of an exposed surface while sputtering the surface of a sample by ion bombardment. Japanese Patent Laid-Open No. 2001-240820 discloses an example of this method.
For a ground surface, the same analysis method as described above is used. In the TOF-SIMS, the primary ion beam power for measurement is increased so that a sample can be sputtered in the depth direction by the ion bombardment. Further, sputtering and measurement are alternatively performed to obtain the depth profile data.
As the primary ions for the TOF-SIMS, cluster ions composed of two or more atoms, not ions composed of a single atom, may be used. Even when a sample surface is bombarded by cluster ions with high acceleration energy, the cluster ions stay at a shallow depth from the sample surface. And the molecules around the cluster ions impact point are ionized and emitted. Therefore, the cluster ions are very useful for TOF-SIMS analysis of an ultra-thin surface layer.
In order for a solid surface to have a water-repellent property, the solid surface is treated by forming a mono-layer using a surfactant copolymer. And the solid surface has hydrophobic groups at the outermost surface. The water-repellency of a solid surface can be estimated by measuring each atom or molecule ratio in the depth direction of an ultra-thin organic layer formed on the solid surface.
However, general sputtering ions, such as argon ions, cesium ions, gallium ions, gold ions, and bismuth ions, work not only for sputtering a surface but also for destroying an internal structure. In particular, in an organic compound mono-layer such as a mono-molecular layer used for water-repellent treatment or a mono-layer of a molecular bonding inorganic compound, the layer structure is destroyed by ion sputtering because the mono-layer has weak bonds on a solid surface.
As a method capable of sputtering an organic sample surface by sputtering without destroying the internal structure thereof, a method of sputtering a surface using fullerene ions has recently been developed. Fullerene ion sputtering apparatuses capable of being mounted on various surface analyzers are used commercially.
Further, a system for cooling a sample stage with liquid nitrogen has been used commercially. Cooling a sample can not only freeze liquid components and volatile components in the sample but also decrease damage due to fullerene ions impact. The structural analysis application of organic compounds in the depth direction by fullerene ion sputtering has been developed more.
In particular, a time-of-flight secondary ion mass spectrometer is one of the few analysis machines getting molecular structure information of molecular compounds such as organic compounds, and is the only one of the analysis machines getting molecular structure data in the depth direction with a sputtering apparatus at the present time.
By using fullerene ions to sputter an organic compound surface, the surface can be sputtered without destroying the internal structure thereof. However, the inventors have found by measurement that fullerene remains as a contamination on the sputtered sample surface.
When fullerene remains on the sputtered surface, it is impossible to distinguish between fullerene contamination data and original surface data even by a surface structure analysis using a time-of-flight secondary ion mass spectrometer, and thus analysis is very difficult.
TOF-SIMS analysis for a surface including a fullerene contamination or an organic compound surface from which the fullerene contamination has been removed has another problem. Namely, an organic compound has a complicated molecular structure, and thus the organic compound does not have a constant density in a solid state and forms a surface in which the density varies in the depth direction. Therefore, it is uncertain how deeply primary ions impact on the surface to emit secondary ions, and thus the precise analysis depth points in the surface cannot be determined. This point significantly distinguishes an organic compound surface from a clean inorganic solid surface and makes TOF-SIMS analysis for an organic compound surface more difficult.